Summary Amblyopia is a visual impairment due to the disruption of binocular vision during early childhood. This disorder affects the function of fine-scale structures within visual cortex, such as ocular dominance columns in the striate visual cortex (V1) and neural columns that comprise magno- and parvo-cellular streams within visual area V2, V3, V3A, V4 and MT. This developmental disorder leads to a variety of perceptual impairments from 2D spatial distortion, to decreased acuity in 3D perception and abnormal global motion perception. Amblyopia is one of the most common visual disorders, affecting approximately 3% of the world?s population (i.e. 10 million people in the US alone). Despite the high prevalence of amblyopia and its severe impact on visual perception, our knowledge of its impact on visual cortical processing is largely limited to findings based on invasive techniques (e.g. single cell recordings) in animals. This is mainly due to technical limitations (e.g. low spatial resolution) of conventional neuroimaging techniques, commonly used in human studies, compared to the size of those cortical structures affected by amblyopia. However, with recent advances in high-resolution neuroimaging techniques, based on using ultra-high field scanners, it is now possible to localize and study functional properties of the fine-scale cortical structures, affected by amblyopia, in humans non-invasively. Using these advanced techniques, we have already provided one of the few direct in vivo evidence for ocular dominance columns in human V1 (Nasr and Tootell, 2016) and depth- (Nasr et al., 2016; Nasr and Tootell, 2016) and color-sensitive (Nasr et al., 2016; Tootell and Nasr, 2017; Nasr and Tootell, 2018) neural columns within human extrastriate visual areas. These findings, plus our preliminary results enclosed with this proposal suggest that, by taking advantage of these modern neuroimaging techniques, neural impairments underlying amblyopia can be revealed and scrutinized directly in humans with a spatial scale comparable to animal studies. During the course of this project, we will use the cutting-edge high resolution neuroimaging technology available to us in the Martinos Center for Biomedical Imaging to reveal impacts of amblyopia on visual system. The results of this project not only fundamentally change the scale at which we can understand the neural mechanisms underlying amblyopia in human, but also 1) introduces new biomarkers that can be potentially used for amblyopia classification and 2) provides new tools to assess effectiveness of amblyopia treatments which are still debated in the absence of any physiological evidence.